1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a surface of a substrate, such as a semiconductor wafer, and more specifically to a polishing apparatus and a polishing method which obtain a film-thickness distribution over the entire substrate surface including a central portion and a peripheral portion thereof during polishing of the substrate and control a load on the substrate based on the film-thickness distribution.
2. Description of the Related Art
A CMP (chemical mechanical polishing) apparatus is widely known as equipment for polishing a surface of a substrate, such as a semiconductor wafer. This CMP apparatus polishes the surface of the substrate by pressing the substrate against a polishing pad on a rotating polishing table while supplying a polishing liquid onto the polishing pad. The CMP apparatus typically has a film-thickness measuring device for measuring a film thickness or a signal equivalent to the film thickness. The CMP apparatus having such a film-thickness measuring device controls a polishing load on the substrate based on a measured value of the film thickness obtained from the film-thickness measuring device and to determine a polishing end point. An eddy current sensor or an optical sensor is generally used as the film-thickness measuring device.
FIG. 1 is a plan view showing a positional relationship between film-thickness measuring device of a conventional CMP apparatus and substrate. A film-thickness measuring device 100 is provided in a polishing table 102 so as to face a substrate W on a polishing pad 105. The film-thickness measuring device 100 measures the film thickness at multiple measuring points on the substrate W while moving across the substrate W each time the polishing table 102 rotates. In the conventional CMP apparatus, the film-thickness measuring device 100 is arranged so as to pass through the center of the substrate W, as shown in FIG. 1. This is for the purpose of measuring the film thickness at multiple measuring points distributed in a radial direction of the substrate W, as shown in FIG. 2.
There exist microcircuit patterns on the surface of the substrate to be polished. In some regions on the substrate, the existence of such circuit patterns could cause a difference in obtained data indicating the film thickness (e.g., voltage value or current value in the case of using the eddy current sensor, relative reflectance in the case of using the optical sensor) even when the film thickness is the same. Thus, in order to avoid such an influence of the circuit patterns, smoothing is performed on the data.
The CMP apparatus determines the polishing loads on multiple regions (e.g., a central portion, an intermediate portion, a peripheral portion) of the substrate based on a film-thickness profile obtained during polishing and polishes the substrate so as to make the film thickness uniform. However, in the conventional CMP apparatus, an accurate film thickness cannot be obtained in the peripheral portion of the substrate because of the smaller number of measuring points on this portion. This problem will be explained with reference to FIG. 2. FIG. 2 is a view showing measuring points on the substrate at which film-thickness measurement is performed while the polishing table makes one revolution. The peripheral portion of the substrate is an outermost annular portion having a width ranging from 10 mm to 20 mm. Because of its narrow width, the number of measuring points on the peripheral portion is small, as can be seen from FIG. 2.
The peripheral portion of the substrate is most likely to be affected by the polishing load and the polishing liquid, and therefore the film thickness is likely to vary greatly during polishing as compared with other regions. Moreover, an initial film thickness in the peripheral portion of the substrate is, in many cases, larger than that in other regions. Thus, it is necessary to accurately measure and monitor the film thickness in the peripheral portion during polishing of the substrate. However, as described above, it is difficult to obtain an accurate film thickness in the peripheral portion because of the smaller number of measuring points on this portion.
FIG. 3 is a graph showing a change in the measured value of the film thickness in the central portion of the substrate and a change in the measured value of the film thickness in the peripheral portion of the substrate. In FIG. 3, a vertical axis represents measured value (estimated value) of the film thickness obtained by the optical sensor, and a horizontal axis represents polishing time. As can be seen from FIG. 3, the film thickness in the central portion (see FIG. 2) of the substrate decreases gradually with the polishing time, while the film thickness in the peripheral portion (see FIG. 2) varies irregularly. This is because the small number of measuring points in the peripheral portion cannot provide sufficient data for the smoothing. In particular, when the polishing table rotates at a high speed, the number of measuring points in the peripheral portion becomes even smaller.
As described above, it is difficult to obtain highly-accurate film-thickness data in the peripheral portion of the substrate, and consequently a highly-accurate film-thickness profile of the substrate cannot be obtained during polishing. As a result, it has been difficult to obtain a desired film-thickness profile through feedback of the film-thickness profile to the polishing load.